The invasion of the New World by West Nile virus has had an unprecedented impact on human, veterinary and wildlife health. The pending invasion of West Nile virus [WN] into areas of California supporting consistent, inconsistent and no St. Louis encephalitis virus [SLE] transmission provides a unique natural experiment to test the hypothesis that similarities in natural history among closely related flaviviruses within the Japanese encephalitis serocomplex preclude concurrent sympatric amplification. WN has now invaded all of the United States east of the continental divide, including areas where SLE was a major public health problem. However, the pattern and efficiency of this invasion and the impact of WN on endemic SLE virus, mosquito vector and vertebrate host populations have not been addressed in a systematic fashion. Tracking the invasion of a new pathogen such as WN into a new environment also has important Bioterrorist relevance, because it simulates the amplification and establishment of a released Category B agent in different epidemiological and ecological settings. Our research is especially relevant for California, which has a large human population, valuable horse industry, extensive agriculture, elevated economic output and highly visible landmarks. California will provide a unique environment for these studies, because 1) previous and on-going studies have investigated the natural history and persistence of endemic arboviruses, including SLE and western equine encephalomyelitis virus [WEE, a category B agent], 2) diverse ecological situations allow the comparison of virus invasion into markedly different habitats ranging from densely populated Los Angeles to rural farmland, and 3) a strong mosquito control and public health infrastructure provides the logistical foundation necessary for in-depth field studies. We plan to test our hypothesis of mutual Flavivirus exclusion by conducting research to address the following aims: 1. Track the invasion, transmission dynamics and epidemiology of WN in cool maritime, hot desert and inland agricultural ecosystems having different host populations and varying levels of endemic SLE and WEE activity. Compare the effectiveness of different surveillance indicators of WN, SLE and WEE amplification [mosquito infection, wild bird infection, dead bird surveillance, sentinel chicken seroconversion] and evaluate prospectively the ability of the California State Mosquito-borne Virus Surveillance and Response Plan to accurately forecast the risk of human infection. 2. Develop a degree-day temperature model for WN replication within representative mosquito host populations to compare the rate of viral amplification [duration of the extrinsic incubation period] and the estimated duration of the transmission season in different environments to previously studied SLE. 3. Investigate factors that may affect WN invasion and persistence, including host competence, cross protective immunity and mechanisms of interseasonal persistence, including vertical transmission in mosquitoes and chronic infections in birds. [unreadable] [unreadable]